In electrostatography an electrostatic image is formed on an image member. For example, the electrostatic image can be formed by imagewise exposing a photoconductive image member. Charged toner particles are applied to the electrostatic image member to form a toner image defined by the electrostatic image. The toner particles can be applied to the electrostatic image member via carrier by a well-known development process, for example, cascade development or magnetic brush development. Toner particles and carrier particles together are referred to as developer. (Toner particles are referred to collectively as "toner" and carrier particles are referred to collectively as "carrier".) Developers are formulated such that the toner and carrier will acquire triboelectrical charges of opposite polarity upon mixing. Because the toner and carrier have opposite charges the toner will cling to the surface of the carrier until the developer is brought into contact with the electrostatic image member. When the developer contacts the electrostatic image member, the electrostatic forces (sometimes in combination with an additional applied field) attract and pull the toner away from the carrier and to the electrostatic image member. After the toner image is formed, usually it is transferred from the electrostatic image member to a receiver onto which it is fixed by known methods, for example, by the application of heat and pressure by a fuser member.
In order for the developer to function properly, the electrostatic attractions between the toner particles and the carrier particles must be strong enough to hold the toner to the carrier while the developer is being transported to and brought into contact with the electrostatic image member. However, the electrostatic attraction between the toner and carrier must also be weaker than the electrostatic forces between the toner particles and the area to be toned on the electrostatic image member, so that the toner will separate from the carrier and attach to the electrostatic image member. Therefore, the electrostatic charge between the carrier and toner should be maintained within an optimum range for sufficient toning of the electrostatic image member to occur.
The toner in developers often contains material referred to as charge agent or charge-control agent which helps to establish and maintain the electrostatic charge on toner within the optimum range. Usually negative charge-control agent materials are used in toners to be charged negatively and positive charge-control agent materials are used in toners to be charged positively, but combinations of negative and positive charge-control agent materials can be used in toners to achieve an overall negative or positive charge on the toners. Many types of charge-control agents are described in published patent literature. For example, see U.S. Pat. Nos. 3,893,935; 3,944,493; 4,834,920; 4,323,634; and 4,394,430.
There are problems with the negative charge-control agent materials disclosed in the background art. Firstly, many negative charge-control agents contain metals, for example, metal complexes of salicyclic acid, metal salts of nicotinic acid and zinc complexes of aromatic hydroxycarboxylic acid and others as disclosed in U.S. Pat. Nos. 4,206,064; 4,789,615; 4,656,112; and 4,624,907. Many metal-containing charge-control agents are toxic; therefore, there are health concerns related to their handling and environmental concerns related to their disposal. Secondly, many of the background art negative charge-control agents are colored. Because of their color, these charge-control agents affect the final color of a toner. The effect of the colored charge-control agents on the toner is a particular problem when fabricating non-black toners, for example, the clear, cyan, magenta and yellow toners.
Other considerations in the selection of charge-control agent materials to be utilized in toners and developers is the effect the charge-control agent will have on a toner's propensity to dust during use and its charge-to-mass ratio (Q/M). In general, dusting increases as the absolute value of the Q/M decreases. Low Q/M levels are highly desirable, because theoretically a larger quantity of toner will attach to an electrostatic image member for a given charge; however, when the charge on the toner is too low the toner often will not adhere to the electrostatic image member and instead behaves like dust, contaminating the machine.
It is toward the objective of providing metal-free, colorless negative charge-control agents, which provide toners and developers exhibiting acceptable dusting and Q/M characteristics, that the present invention is directed.